WHAT SCIENCE AND SUPER-ACHIEVERS TEACH US ABOUT HUMAN POTENTIAL

The book

The author

David Shenk is the national bestselling author of five previous books, including The Forgetting ("remarkable" - Los Angeles Times), Data Smog ("indispensable" - New York Times), and The Immortal Game ("superb" - Wall Street Journal). He is a correspondent for TheAtlantic.com, and has contributed to National Geographic, Slate, The New York Times, Gourmet, Harper's, The New Yorker, NPR, and PBS.

March 22, 2007

The nature and nurture of muscles

Some of the truly fascinating insights into talent and greatness emerge from the realm of human musculature -- how our skeletal muscles are initially formed, the attributes of different muscle fibers, and the different ways muscles can be transformed by activity and training. Reviewing the nature/nurture of muscles is also perhaps the best window into the dynamics of genetic expression. Here's an overview:

This overview concentrates on skeletal muscles -- the muscles we exert direct control over.

What are the basic components of skeletal muscle?Each skeletal muscle is a bundle of thousands of specialized elongated cells called muscle fibers.

The fibers are fed by tiny, blood-filled capillaries, held together with various kinds of connective tissue, and fired ("innervated") by motor neurons -- one neuron firing 600 or so muscle fibers.

Each individual muscle fiber also contains a string of DNA-containing nuclei positioned just underneath and along the entire length of its membrane. The genetic material constantly instructs each fiber how to react *and adapt* to various circumstances.

There are two basic types of muscle fibers:--
"Slow-twitch" (type I) fibers are designed to contract for long periods
of time; packed with mitochondria, they are extremely efficient at
converting oxygen to fuel. ----- These fibers enable us to jog, swim, bicycle, and other lengthy activities

-- "Fast-twitch" (type II) fibers contract rapidly and forcefully
for a period of seconds, very quickly using voracious amounts of
(anaerobic) energy, becoming spent and
needing to rest and replenish. ----- These fibers enable us to sprint, jump, lift weights and other short-burst activities.

In musculature, we are not all created equal
Although on average, human beings have about a 50/50 mix of slow and
fast-twitch muscle fibers, some are born with differing proportions.

"The 'average' healthy adult has roughly equal numbers of slow and
fast fibers in, say, the quadriceps muscle in the thigh. But as a
species, humans show great variation in this regard; we have
encountered people with a slow fiber percentage as low as 19 percent
and as high as 95 percent in the quadriceps muscle." (Anderson et al, 2000)

As anyone might logically expect from the above description of the fiber
types, a higher proportion of one or another can offer certain potential advantages to highly-trained
athletes. Elite marathon runners and cyclists benefit from a higher
proportion of slow-twitch fibers, for example, while sprinters benefit from a higher
proportion of fast-twitch fibers. (Anderson et al, 2000).

These genetic differences, however, must be put into careful context.First, muscle fiber proportion is only one of many performance factors. On its own, it is not a good predictor of individual performance.

Second, muscles are tremendously adaptive to external stimulus, and are designed to be so. The muscles we are born with are merely default muscles -- ready and waiting to recreated in one or another particular direction by active use.

***

To understand how adaptation is literally built into our muscle DNA, let's look at all the things that happen as a result of trainingAt any given time, each muscle is adapted to a status quo of activity and exertion -- i.e., each muscle is exactly as big, strong and efficient as it needs to be. When pushed just beyond the ordinary level of exertion, a
number of physiological changes begin to unfold:1. Neural response.

"The first measurable effect is an increase in the neural drive
stimulating muscle contraction. Within just a few days, an untrained
individual can achieve measurable strength gains resulting from 'learning' to use the muscle." (NSMRC)

2. Genetic response makes muscle fibers more efficient. In response to extended (aerobic) exercise -- e.g. jogging -- there is a genetic response in the nucleus of each cell fiber that makes it more efficient and enduring: increasing the number of mitochondria and provoking an increase in surrounding capillaries and the accumulation of fats and carbohydrates. (Wiki)

3. Genetic response makes muscle fibers become stronger and grow in size.In response to overload/resistance exercise -- e.g. weight lifting -- the DNA responds with instructions that will lead to the strengthening and enlarging [hypertrophy] of each fiber.

"As the muscle continues to receive increased demands...upregulation appears to begin with the ubiquitous second messenger system (including phospholipases, protein kinase C, tyrosine kinase, and others). These, in turn, activate the family of immediate-early genes, including c-fos, c-jun and myc. These genes appear to dictate the contractile protein gene response. "Finally, the
message filters down to alter the pattern of protein expression. It can
take as long as two months for actual hypertrophy to begin. The
additional contractile proteins appear to be incorporated into existing myofibrils
(the chains of sarcomeres within a muscle cell). ...These events appear to occur within each muscle fiber.
That is, hypertrophy results primarily from the growth of each muscle
cell, rather than an increase in the number of cells."
(NSMRC)

4. When training is particularly intense and prolonged, slow-twitch muscle fibers can become transformed into fast-twitch fibers, and vice-versa.

Exercise, stretches and other muscle activity (LEFT) interacts with DNA in the nucleus (CENTER, circled in red), which in turns interacts with protein translators to effect changes on the cell and surrounding tissue (RIGHT).

While evolution has given humans some variability in muscle types, the much more powerful product is its adaptivity. Muscles are designed to be rebuilt.

"The ability of striated muscle tissue to adapt to changes in activity
or in working conditions is extremely high. In some ways it is
comparable to the ability of the brain to learn." (Bottinelli and Reggiani, 2006)

Comments

This is off-topic, but...

In case you haven't already, you might want to have a look at the book "The Brain that Changes Itself," by Dr. Norman Doidge. It contains several fascinating case studies on neuroplasticity. I think you have already posted on a couple of the cases.

During the same trip to the bookstore, I browsed through another book titled "Talent is Never Enough," (author forgotten), which turns out to be more inspirational than expository.

Genius, savants, prodigies ... topics I love to keep an eye on. As you push forward in your research, you may find it helpful to peek in at my del.icio.us bookmarks - I might catch something you haven't seen yet. A few months ago I went through every single del.icio.us article tagged as "genius" and marked the most relevant pieces.